Microroughness is defined as "surface roughness components with spacings between irregularities (spatial wavelength) less than about 100 micrometers." This definition differentiates microroughness from the larger scale surface variations of bow and warp, which have spatial wavelengths typically on the order of several millimeters.
The very small levels of surface texture associated with microroughness are becoming more problematical in a number of industries as the complexity of integrated circuits and the amount of information stored on disk drives increases. As an example, geometries in the integrated circuit industry are fast approaching molecular dimensions. The June 1994 report, the National Technology Roadmap for Semiconductors (NTRS), has published a requirement for gate oxide thicknesses approaching 4.5 nm.+-.4%. As a point of reference, the lattice constant for lightly doped (i.e., nearly pure) silicon is 0.543 nm. The gate dielectric molecule, silicon dioxide, is nominally 0.355 nm "diameter" (based on the cube root of the volume ratio). The ability of silicon dioxide or any film layer to function efficiently as an insulator depends partially on the underlying microroughness of the silicon surface. For oxides less than 10 nm, breakdown voltages are reduced commensurately with increased levels of microroughness. This can be readily understood by envisioning the "peaks" of the microroughness terrain as being much closer to the film surface than the overall average level of the peaks and valleys combined. Additionally, there are similar effects on film layers deposited in later processing steps, and an effect on bonding for silicon-on-insulator (SOI) applications.
Currently, there are several techniques available for measuring microroughness. However, the results tend to be qualitative. Until recently, there was no metrology standard available to correlate the accuracy of various instruments. This becomes especially important when comparing instruments with differing spatial bandwidths, each possessing a unique transfer function. Due to the varying spatial bandwidths, different types of instruments can give rms microroughness values that differ by over an order of magnitude, even when measuring the same surface.
An object of the invention is to provide a calibration standard used to verify the accuracy and precision of analytical test equipment for measuring microroughness, and thereby allow such equipment to provide absolute quantitative values based on such a standard instead of the relative qualitative results that are presently all that is available.